Yes you can get to Narita airport by 6pm from Niseko on the same day comfortably.
The earliest scheduled bus leaves for Chitose airport from Niseko at 8:10 and arrives Chitose 11:00am.
Several airlines (Jetstar, Air Asia, JAL and ANA) operate flights from Chitose to Tokyo's International Narita airport.
There are also frequent flights from Chitose to Tokyo's domestic Haneda airport. The flight time is 90 minutes. From Haneda to Narita airport it's a 90 minute bus transfer which costs 3,000yen per-person. No reservations required. These buses (called Airport limousine) also leave very frequently.
If you manage to get a flight departing from Chitose to Haneda at 12:00, you will be able to get to Narita by 4:00pm comfortably. So a 6pm International departure from Nartia would be the earliest if you were to fly into Haneda from Chitose.
Lastly there is an alternative to the coach transfers from Niseko to Chitose airport which can get you to Chitose when ever you wish. Private transfers are 35,000yen one way for up to 5 passengers. And 45,000yen one way for up to 9 passengers.